Solid-state radiation transducers (SSRTs), e.g., light-emitting diodes (LEDs), organic light-emitting diodes, and polymer light-emitting diodes, are used in numerous modern devices for backlighting, general illumination, and other purposes. FIG. 1 is a partially-schematic, cross-sectional view of a conventional LED device 100 having a lateral configuration. As shown in FIG. 1, the LED device 100 can include a growth substrate 102 under an LED structure 104 with an active region 106 positioned between an N-type layer 108 and a P-type layer 110. The device 100 can also include a first contact 112 electrically connected to the P-type layer 110 and a second contact 114 electrically connected to the N-type layer 108. As shown in FIG. 1, the second contact 114 extends across only a small portion of the N-type layer 108. This type of limited connection between the second contact 114 and the N-type layer 108 can cause poor current spreading within the N-type layer 108, especially when the N-type layer 108 includes N-type gallium nitride, which has relatively low lateral conductivity. Poor current spreading can cause portions of the device 100 to be underutilized and can lower the lumen output and/or the efficiency of the device 100.
FIG. 2 is a partially-schematic, cross-sectional view of another conventional LED device 200 having a vertical configuration that can have enhanced current spreading relative to the device 100 of FIG. 1. The device 200 includes a carrier substrate 202 and an LED structure 204 with an active region 206 positioned between an N-type layer 208 and a P-type layer 210. During formation of the LED device 200, the N-type layer 208, the active region 206, and the P-type layer 210 can be formed sequentially on a growth substrate (not shown) similar to the growth substrate 102 shown in FIG. 1. A first contact 212 can be formed on the P-type layer 210, and the carrier substrate 202 can be attached to the first contact 212. The growth substrate can then be removed and a second contact 214 can be formed, e.g., in a pattern, on the N-type layer 208. The device 200 can then be inverted to produce the orientation shown in FIG. 2. As shown in FIG. 2, the second contact 214 extends across a significant portion of the N-type layer 208. This can facilitate current spreading within the N-type layer 208 resulting in improved lumen output and/or efficiency of the device 200. In the vertical configuration shown in FIG. 2, however, the second contact 212 can disadvantageously interfere with emissions from the LED structure 204. The footprint of the second contact 212 can be reduced, e.g., to a series of lines as shown in FIG. 2, but cannot be made insignificant in this configuration without sacrificing the enhanced current spreading.
FIG. 3 is a partially-schematic cross-sectional view of another conventional LED device 300 having a buried-contact configuration in which the second contact can facilitate enhanced current spreading relative to the device 100 of FIG. 1 while interfering less with device emissions than in the device 200 of FIG. 2. Similar to the device 200 of FIG. 2, the device 300 can include a carrier substrate 302 and an LED structure 304 having an active region 306 positioned between an N-type layer 308 and a P-type layer 310. Also similar to the device 200 of FIG. 2, the device 300 can include a first contact below the P-type layer 310. As shown in FIG. 3, the device 300 can include a second contact 314 with buried-contact elements 315 that extend through the first contact 312, the P-type layer 310, and the active region 306, and partially into the N-type layer 308. A dielectric layer 316 can electrically isolate the second contact 314 from the first contact 312, the P-type layer 310, and the active region 306. The second contact 314 can be electrically connected to the N-type layer 308 at multiple transition regions 318 distributed across the area of the N-type layer 308. Distributing the transition regions 318 can enhance current spreading within the N-type layer 308. Furthermore, since much of the second contact 314 is below the P-type layer 310, the second contact 314 can interfere less with emissions from the LED structure 304 than the second contact 212 interferes with emissions from the LED structure 204 of the LED device 200 shown in FIG. 2.
Although the buried-contact configuration shown in FIG. 3 is an improvement in many ways relative to the lateral configuration shown in FIG. 1 and the vertical configuration shown in FIG. 2, it still has significant drawbacks. For example, many modern LED devices include color-converting materials, e.g., phosphors, positioned in the path of emissions from an LED structure. Color-converting materials can absorb light that an LED structure emits at a certain wavelength range and emit light at a different wavelength range. Color-converting materials typically release light in all directions, including back toward the LED structure. Reflection of the light emitted back toward the LED structure can be an important factor in determining lumen output and efficiency of a device. The uniformity of this reflection also can be important, particularly in display and projection applications. Conventional buried contacts typically interfere with this reflection and/or other performance-related reflection to some degree. For example, the transition regions 318 shown in FIG. 3 can absorb light and cause undesirable dark spots in the near field. For this reason and/or other reasons, there is a continuing need for innovation with regard to SSRT devices, such as to improve the lumen output, efficiency, and output uniformity of buried-contact SSRT devices.